Conventionally, a method for recovering and removing acid gases, in particular, CO2, which are contained in gases (gases to be treated) such as various industrial gases produced in chemical plants, such as a natural gas and a synthesis gas, and flue gases, has been studied and various methods have been proposed.
For example, for the flue gases, a method for allowing CO2 in a flue gas to come into contact with an alkanolamine solution or the like to remove and recover CO2, and a method for storing recovered CO2 without discharging CO2 to the atmosphere have been actively studied.
Examples of the alkanolamine that can be used include monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), 2-amino-2-methyl-1-propanol (AMP), diisopropanolamine, 2-methylaminoethanol, 2-ethylaminoethanol, 2-propylaminoethanol, n-butylaminoethanol, 2-(isopropylamino)ethanol, and 3-ethylaminopropanol.
For example, an absorbing liquid formed of a mixture of secondary amine or a mixture of secondary amine and tertiary amine is disclosed in a United States patent specification. It is proposed that this mixed absorbing liquid is an advantageous absorbing liquid because an absorbing ability and regeneration energy are significantly improved in the mixed absorbing liquid compared with an MEA single absorbing liquid (PTL 1).
In the case in which a monoethanolamine (MEA) absorbing liquid is used, there is a problem in that degradation in the absorbing liquid severely progresses because of oxygen or the like in a gas.
Therefore, conventionally, a method has been proposed for stabilizing an absorbing liquid by adding, for example, trialkanolamine or methyldiethanolamine (MDEA) to the absorbing liquid (PTL 2 and PTL 3).